The invention is based on a priority application EP 02 360 158.6 which is hereby incorporated by reference.
Generally speaking, optical wavelength multiplexing and demultiplexing have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with an optical grating comprising a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbor by a predetermined amount. The grating is connected to the input of a second star coupler, the outputs of which form the outputs of the switching, multiplexing, and demultiplexing apparatus.
The geometry of such an interconnection apparatus may be such that a plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, an input wavelength is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelength also permits switching between any selected input port to any selected output port.
Accordingly, these devices are generally referred to as frequency routing devices and more specifically wavelength division multiplexers (WDM).
Ideally, the operation of these AWG and WDM optical devices should by predictable and consistent over a wide range of environmental conditions. Unfortunately however, in practice, the operational performance of such devices is significantly affected by variations in the temperature of the environment surrounding the device.
More specifically, the wavelength spectrum of existing AWG designs shifts with temperature (T) for at least two reasons. First, where n represents the refractive index of the waveguide material, dn/dT≠0 and secondly, the thermal expansion, i.e. dL/dT, where L represents length, likewise does not equal zero.
To date, some of the techniques used to create optical devices that are less sensitive to temperature changes have included using a heater with a temperature controller to stabilize the wavelength spectrum of the AWG. Unfortunately, such a design is expensive and impractical in applications where electrical power is not readily available. In addition, the semiconductor art has demonstrated a temperature insensitive semiconductor WGR that includes a waveguide region with different dn/dT.
However, even in light of the technical advances mentioned above, there remains a definite need for a practical design and method for making optical waveguide filtering devices temperature independent. This is particularly true for compensating waveguide grating routers, which are, at present, the multiplexers of choice for dense WDM systems. Furthermore, given that silicon optical bench routers are now components of various Next Generation Lightwave Networks (NGLN) and are planned for use in Fiber-to-the-Home (FTTH) access networks, temperature-compensating optical devices like AWGs are highly desirable for these networks. In the U.S. Pat. No. 6,137,939 a design is proposed where a portion of the length of at least one waveguide within the optical device is modified in a way that stabilizes the wavelength spectrum passing therethough. The solution is to fill an elastomer material in either several blocks incorporated in the selected waveguides or to incorporate a wedge filled with the said elastomer material.
Also known are several designs of wedges from U.S. Pat. No. 6,304,687. All this proposed designs allow to adapt the WGR to different temperature with an acceptable insertion loss. But the known structures introduce a cross talk penalty in the WGR which disturbed the channels and mismatches the WGR for the several uses. To reduce the effects of cross talking between the waveguides it is known to use a design which separates the channels by implementing several separated wedges of same width of the material into the waveguides which has opposite temperature coefficient to the silica waveguide. The use of this wedges with different lengths but same width do not suppress the cross talk effects in a sufficient way.